LLEXT-related files are currently scattered in multiple locations in the
build directory. For easier access, this patch groups the outputs in a
subdirectory named 'llext' at the root of the build binaries, alongside
the 'zephyr' directory. This directory will thus contain the generated
debug ELF and the final .llext file for each compiled extension.
Note that this does not affect out-of-tree projects that use LLEXT,
since they already pass the full LLEXT file path to add_llext_target().
Signed-off-by: Luca Burelli <l.burelli@arduino.cc>
8660020205